Theresa May said there would be no special measures put in place at Britain’s ports when Romanians and Bulgarians gain full rights to work in the UK Photograph: PA

Theresa May was late Monday distancing herself from Home Office plans to impose a cap on migration from the EU's existing member states after Nick Clegg warned that such a move would be "illegal and undeliverable".

As the deputy prime minister warned that some Tories were damaging Britain's interests by "flirting" with a British exit from the EU, the home secretary said a cap would apply only to future members of the EU.

The clarification, in response to the alleged leaking by the Home Office to the Sunday Times of a government report that raised the prospect of a cap, came as May said there would be no special measures put in place at Britain's airports and ports in a fortnight's time when Romanians and Bulgarians win the full rights to work in the UK.

Her pledge that it will be "business as usual" at Britain's borders on 1 January comes as an ICM poll for British Future show that 38% of Britons expect up to 50,000 Romanians and Bulgarians to come to Britain to work next year. The survey shows that some of the more extreme sustained media coverage has had such an impact that 13% of British voters now believe that as many as 500,000 Bulgarians and Romanians could come to work in Britain in 2014.

The survey was published as David Cameron indicated that the Tory manifesto for the next election would include plans to curb the free movement of workers within the EU. Speaking during a day trip to Afghanistan, the prime minister said: "On immigration reform more broadly, Theresa May and I are both committed to delivering net migration in the tens of thousands and I think it's important as we go towards the next election that we look at all the ways that we can make sure we can achieve that and obviously there's the issues of what we put in the election manifesto and as I've said I think we need to open up the whole area of European obligations and how free movement works. I'm in favour of people being able to go and work in different European countries – many British people take advantage of that. But clearly recent, the recent joinings, particularly what happened with the Polish situation, I think, means that we do need reform as I said in my article in the Financial Times. So we should consider what are the best ways of achieving that. There will be an important discussion between now and the next election."

In his FT article last month the prime minister raised the prospect of imposing a cap on migration from future EU members until their economies have reached a certain level of income per head. No 10 sources indicated that the prime minister still believes a cap would only apply to future EU members, though some cabinet ministers are likely to argue that it should be introduced for existing member states in line with the leaked government report which quotes David Goodhart, the writer on immigration, who proposed an emergency cap of 75,000.

Clegg hit out at the proposal amid irritation within No 10 over the alleged orchestration by May's office of the leaking of the report. He said: "My advice to the Home Office is to spend less time leaking policies that are illegal and undeliverable and spend more time delivering on the policies that we have agreed as a coalition government, notably the reinstatement of exit checks.

"I remain very frustrated that the Home Office has still not delivered something which I personally insisted should be in the coalition agreement, which is a reinstatement of the checks which allow us to know not only who is coming into this country but who is leaving as well."

Speaking after the intervention by Clegg, the home secretary insisted she was only talking about possible new conditions on unspecified future members of the EU. May told the commons home affairs select committee: "It is right to say we should look at the accession treaties for new countries coming into Europe and within that we should look at the question as to whether we should have greater flexibility rather than just a period of time for transitional controls. Maybe controls should be in place until the national income of a country has reached a certain percentage of the country's national income or indeed if migration reaches a certain level whether there's a possibility in those circumstances to introduce a cap."

The home secretary said she agreed that Clegg had made his claim that such a move would be illegal under the EU treaties if a 75,000 a year cap on EU migration was introduced now. "He has, as I understand it, made a statement on the basis of if we were going to do this now, this is what the situation would be. What I'm saying is, I'm not proposing to do it now. We're talking about potential reforms of accession treaties for the future."

The home secretary's remarks represent a change from her stance earlier this month when she indicated that Britain would seek to impose a cap on existing EU member states in the future. In a statement on the eve of the EU justice and home affairs council in Brussels, she said: "Looking ahead, we must seize the opportunity presented by the prime minister's plan to reform the EU and address the problems caused by free movement. For example, why shouldn't national governments be allowed to impose a cap on numbers if European immigration reaches certain thresholds?"

May is taking a close interest in the document leaked to the Sunday Times – the migration section of the government's review of the EU's balance of competences – as she faces pressure on two fronts. In the first place ministers are nervous about the prospects of large numbers of Romanians and Bulgarians travelling to the UK from 1 January when restrictions on the right of their citizens to work in Britain, in place since they joined the EU seven years ago, are lifted. In the second place ministers are concerned that they will struggle to meet David Cameron's target of reducing net migration to the tens of thousands in the face of rising migration from the EU.

May told the Commons home affairs committee that there was no official forecast of the numbers likely to arrive from Romania and Bulgaria and reminded them that Britain was not alone in lifting its labour market restrictions on the two EU states seven years after they joined with Germany and France amongst the eight other countries opening their borders to them. She said it would be "business as usual" at Britain's borders on January 1.

May said that she had raised a number of ideas about how the transitional controls on new member states could be extended beyond the current seven year time period. She also faces a potential revolt by more than 60 Tory backbenchers who want to extend the current restrictions on Romanians and Bulgarians.

A new migration threshold on future EU members could be set at a percentage of average EU GDP - possibly 75% of Britain's - to determine who could be allowed into Britain to work. The Joint Council for the Welfare of Immigrants warned that one of her proposals to limit migration from poorer countries that had less than 75% GDP could block Britons working in Germany as the UK GDP was only 70% of that of Germany.

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